An overhead gas showerhead with many small gas inlet orifices can cause plasma arcing when used as an electrode to capacitively couple plasma source power. Plasma tends to enter into many of the gas outlet orifices of the electrode and arc inside each orifice. The arcing can melt or sputter metal atoms from the electrode, creating contamination in the plasma and thereby causing the plasma process (e.g., a plasma-enhance reactive ion etch process carried out on a semiconductor workpiece) to fail. Moreover, such arcing damages the electrode by widening different orifices, thereby distorting gas flow distribution at the electrode surface. Finally, if the electrode is metal and is covered with a semiconductor protective layer, such arcing further damages the electrode or generates contamination by attacking the bonding adhesive placed between the semiconductive protective layer and the metallic electrode. A very narrow gas outlet orifice diameter was employed in order to prevent migration of plasma into the gas outlet orifices, but this actually worsened the arcing problem. This was because the greater pressure occasioned by the narrowing of the orifices promoted arcing. Moreover, such narrow orifices were difficult to clean, so that residue from the plasma (e.g., polymers) accumulated within the gas outlet orifices.
Our efforts to avoid arcing in an overhead VHF source power electrode/gas showerhead led to the concept of configuring the gas outlet orifices in the overhead electrode as narrow annuli, which is disclosed in the parent application referenced above. The gas pressure was dropped well above the gas outlet orifices by extremely narrow internal pressure-dropping orifices extending in a radial direction. The arcuate or circumferential length (in the plane of the electrode) of each annular gas outlet orifice enhanced the gas flow conductance within the orifice, which minimized the gas pressure within the high electric field existing at the surface of the electrode. This feature reduced the tendency of gas in the orifices to arc. The narrow width of each annular orifice increased the rate at which the electric field dropped inside the orifice as a function of axial height, so as to confine the high electric fields near the bottom of the gas outlet orifices and away from the upper regions of the electrode where the narrow pressure-dropping orifices were located. This feature minimized the electric field near the upper region of the electrode where the gas pressure dropped from a very high to a very low pressure, thus avoiding coincidence of a high gas pressure and a high electric field in the same location, in order to better suppress arcing.
Such annular-shaped gas outlet orifices require complex machining to fabricate, and are not readily adapted to a curved topology. Therefore, such gas distribution electrodes are essentially confined to a flat shape to avoid excessive fabrication costs. This is particularly true in the case where a semiconductor protective layer covers the bottom surface of the electrode, requiring formation of mutually aligned annular gas outlet orifices in the metal electrode and the semiconductor protective layer.
We have found that in a reactor of the type disclosed in FIGS. 1–30, the plasma ion density distribution can be slightly center high, with low plasma density and low etch rate prevailing at the wafer periphery. In some cases, the plasma ion density at the wafer edge may be 55% or less than the plasma ion density at the wafer center. The etch rate is similarly depressed at the wafer edge relative to the wafer center. There is a need for an overhead gas distribution source power electrode capable of improving plasma uniformity while retaining the advantages described above of low arcing tendency. One way of correcting for a center-high plasma ion density distribution is to configure the electrode surface in an arcuate shape, such as a dome shape or a multi-radius dome shape. However, the complexity of the machining steps required to fabricate an electrode having a low tendency to arc (i.e., one with the annular shaped gas outlet orifices described above) cannot be realized in an arcuate shape, or at least not a shape having a significant curvature. For example, in an electrode that is on the order of 300 mm in diameter, it would not be cost-effective to provide a curvature having more than a few millimeters deviation from center to edge. Such a small curvature may be inadequate to correct or significantly improve a 55% deviation in plasma ion density non-uniformity. The problem is how to provide sufficient curvature in the electrode without increasing the tendency for arcing to occur within the gas outlet orifices.